1. Field of Invention
The invention relates to an asymmetry compensator, and in particular to an asymmetry compensator, which can be utilized in a Partial Response Maximum Likelihood (PRML) Decoder for adjusting the gains and the offsets of asymmetric components of the regenerating signal read from an optical recording medium.
2. Related Art
In general, the optical recording medium is utilized to record the data on the disk by optical means, and to read the data stored on the disk by optical means. As such, the device utilized to read the data stored on the optical recording medium is usually referred to as a signal regenerating device, it mainly comprises: a read/write head for reading/writing the data, a preamplifier, a waveform equalizer, a data detecting circuit, and a decoder.
Usually, in reading the data stored on the optical recording medium, the value of the data is determined by the size of the burned area. However, when the burned area is exceeding ‘large’ or exceeding ‘small’, the distinct asymmetry will appear in the optical disk regenerating signal, which affects the error occurrence rate of the regenerating signal of the optical disk, for example, PRSNR, SbER, which is utilized as an important indicator for indicating the optical disk error rates. Among them, PRSNR is disclosed by NEC, SbER is disclosed by Toshiba of Japan respectively.
U.S. Pat. No. 6,324,144 discloses a technology of eliminating the asymmetry, wherein the difference of numbers of the positive and negative signal is utilized to adjust the offset of the regenerating signal. However, to PRML, the adjustment of the offset is not able to entirely compensate the asymmetry in the signal. In addition, if this asymmetry were severe, the application of this technology would increase the difficulties in discriminating the signal.
U.S. Pat. No. 6,754,160 discloses another technical means, wherein the offset is obtained by comparing the signal of PRML with the regenerating signal of the optical disk. However, this means is less sensitive to the offset, and it needs additional hardware to record the regenerating signals, thus increasing the hardware complexity and expense.
U.S. Pat. No. 6,798,363 discloses another technical means, wherein in addition to adjusting the offset by the difference of numbers of the positive and negative signal, the offset is further adjusted by the symmetry of the short T signal. However, to the PRML, the adjustment of the offset alone is not capable of compensating the asymmetry entirely, for it needs the signal of the Viterbi decoder as the feedback signal, thus making the realization of the hardware even more difficult.
U.S. Pat. No. 6,693,863 discloses another technical means, in which the compensation of the asymmetry is achieved by adjusting the quantified level of A/D. However, the utilization of the analog circuit tends to have the problem of adjustment difficulties.
In addition, in the early open published No. 20030169665 discloses a technology, which is utilized to integrate the gain adjuster and the equalizer as a single unit, thus to reduce the hardware required, and the gain is adjusted by making use of the Viterbi feedback signal. However, the integration of gain adjuster and equalizer as a single unit increases the complexity of the circuit design, yet the offset has not been adjusted.
In other documents related to this subject, for example, as disclosed in “A study of Asymmetry Compensation for Partial-Response Maximum-Likelihood Detection in Optical Recording Media” (Sony paper, 1998), the offset of the signal is obtained by making use of the successive 6T, and then it is transmitted to the Viterbi decoder. However, the signals of succession are required, when it is not the case, then the correct offset can not be obtained. In another related document “Decision-directed adaptive nonlinear canceller for optical read channels” (Cirrus paper, 2001) is disclosed a technical means, the compensation signal output is obtained through the equations, yet in this process a set of predetermined input digital signals are required. In other related documents, for example, “Adaptive Signal Processing Method Using PRML For High Density Optical Disks” (Hitachi Ltd. 2002 IEEE), “Combined adaptive controlled PRML signal processing for high density optical disk” (Toshiba Corp. 2002 IEEE), “Adaptive Partial-Response Maximum-Likelihood Detection in Optical Recording Media” (Sony Corp. 2002 IRRR), the asymmetry of the RF (optical disk regenerating signal) is disclosed, and the solution of the asymmetry is by varying the level of the maximum-likelihood decoder. However, by doing so other problems are derived and have just yet to be solved, such as the convergence, the stability, and the interval between level and level of the optical disk regenerating signal.
To solve the asymmetry problem of the optical disk regenerating signal, the adjustment of the offset or the adjustment of the gain is utilized in the prior art, yet the asymmetry correction of the signal must be achieved by offset adjustment and gain adjustment simultaneously, as such to obtain the correct and precise regenerating signal. Nevertheless, for this problem, the prior art does not have proper solution.